The present invention relates to chassis construction in general, and in particular to a composite metal-and-plastic chassis for electronic apparatus.
Chassis constructed principally of sheet metal have long been used for electronic devices for a number of mechanical and electrical reasons. First, the chassis may be utilized structurally as part of the device frame, and such chassis generally provides a base onto which components and sub-assemblies may be mounted. Electrically, the chassis provides a convenient source of ground potential, and may also provide shielfing between electrical elements, e.g., components and circuit boards, mounted thereon.
From a manufacturing point of view, metal chassis construction represents a number of problems. Preparation of the sheet metal, including shearing, punching, bending, and welding, generally requires expensive tooling and equipment, and is a time-consuming process. Tolerances closer than .+-.0.015 inch are difficult to maintain in the metal-preparation process. Mechanical hardware, such as screws, nuts, washers, brackets, posts, etc., is used to assemble the chassis to other frame members and to mount components and sub-assemblies on the chassis. Accordingly, the parts and hardware count is large, assembly time can be lengthy, and in situations in which automatic hardware insertion machines are employed to install captive hardware, wrong or missing parts and parts mounted in the wrong locations may not be discovered until a time wherein rework would be tedious or time consuming.
It has also been known to construct electronic apparatus housings and cabinets of injection-molded plastic as a way of lowering manufacturing costs. This is particularly true in the case of small or portable devices, such as television sets and certain test and measurement instruments. The cost of molded plastic parts is low. Many parts, such as brackets, holders, and spacers, may be molded in, thereby eliminating many separate parts, reducing final assembly time, and reducing the hardware count. Moreover, much tighter tolerances may be achieved over large areas with the molding process, providing greater precision of parts. However, inclusion of captive hardware is almost as troublesome with a plastic base part as it is with a sheet metal chassis because human errors in the molding process are frequent, e.g., errors such as leaving parts out of the mold and improper sonic insertion. Electrically, plastics are insulators and therefore cannot be used to provide circuit grounding or electrical shields. Attempts to metallize the plastic, such as by plating or applying metallic paints, lead to further problems and expense.